In order to prevent from a short circuit on a light string, each light bulb holder (1) of the light string is provided with a plate (11), as shown in FIG. 1 or as described in U.S. Pat. No. 5,707,138. The plate (11) is perpendicular to the opening of the holder (1) and separates the inner space into two areas and forms two small holes (12) at the end of the holder (1), as shown in FIG. 4, for draining. In order to assemble the structure with the electrical wire (3) and conductive plate (31), the electrical wire (3) is inserted through the hole (12) to engage the conductive plate (31). Then, the electrical wire (3) together with the plate (31) is pulled back into a proper position in the light bulb holder (1). The wire (3) with the plate (31) cannot be inserted into the holder (1) because the width of the plate (31) is larger than the diameter of the small hole (12) of the holder (1). Such assembling procedures of the known light bulb holder is troublesome and inconvenient.